An internal combustion engine supplies a combustion chamber with fuel and air and burns the fuel in the combustion chamber to output a drive force. When burning fuel in the combustion chamber, the air-fuel mixture of the air and fuel is compressed in state. It is known that the compression ratio of the internal combustion engine has an effect on the output and fuel consumption. By raising the compression ratio, it is possible to increase the output torque or reduce the fuel consumption. On the other hand, if making the compression ratio extremely high, it is known that knocking or other abnormal combustion occurs.
Japanese Patent Publication (A) No. 2000-230439 discloses a self-ignition type internal combustion engine which provides a combustion chamber with a sub chamber which is communicated through a pressure regulator, wherein the pressure regulator has a valve element and a valve shaft which is connected to the valve element and is biased to the combustion chamber side. It is disclosed that this self igniting type internal combustion engine pushes up the pressure regulator against the pressure of an elastic member and releases the pressure to the sub chamber when overly early ignition etc. causes the combustion pressure to exceed a predetermined allowable pressure value. This publication discloses a pressure regulator which operates by a pressure larger than the pressure which occurs due to overly early ignition etc. Further, in this publication, an internal combustion engine is disclosed where a sub chamber is formed which communicates with the combustion chamber and a sub piston is inserted able to move vertically in the sub chamber. The sub piston is pressed against by a mechanical spring. It is disclosed that when the fuel is burned, the pressure of the combustion chamber causes the mechanical spring to be compressed and the sub piston to rise and the volume of the sub chamber which communicates with the combustion chamber becomes larger.